Content

Abstract:
Gravitational-wave detectors can search for yet-undiscovered ultralight
bosons, including those conjectured to solve problems in particle physics,
high-energy theory and cosmology. Ground-based instruments could probe boson
masses between $10^{-15}$ eV to $10^{-11}$ eV, which are largely inaccessible
to other experiments. In this paper, we explore the prospect of searching for
the continuous gravitational waves generated by boson clouds around known black
holes. We carefully study the predicted waveforms and use the latest-available
numerical results to model signals for different black-hole and boson
parameters. We then demonstrate the suitability of a specific method (hidden
Markov model tracking) to efficiently search for such signals, even when the
source parameters are not perfectly known and allowing for some uncertainty in
theoretical predictions. We empirically study this method's sensitivity and
computational cost in the context of boson signals, finding that it will be
possible to target remnants from compact-binary mergers localized with at least
three instruments. For signals from scalar clouds, we also compute detection
horizons for future detectors (Advanced LIGO, LIGO Voyager, Cosmic Explorer and
the Einstein Telescope). Among other results, we find that, after one year of
observation, an Advanced LIGO detector at design sensitivity could detect these
sources up to over 100 Mpc, while Cosmic Explorer could reach over $10^4$ Mpc.
These projections offer a more complete picture than previous estimates based
on analytic approximations to the signal power or idealized search strategies.
Finally, we discuss specific implications for the followup of compact-binary
coalescences and black holes in x-ray binaries. Along the way, we review the
basic physics of bosons around black holes, in the hope of providing a bridge
between the theory and data-analysis literatures.